Documentation spelling alignment: centre -> center
Documentation spelling alignment: centre -> center

In the man page, change the single occurence of "centre" frequency to
"center" frequency, as the AE spelling is used in all other places
except for this one.

cbagwell/sox,davel/sox,Distrotech/sox,CaptainHayashi/sox,pcqpcq/sox,Distrotech/sox,CaptainHayashi/sox,uklauer/sox,davel/sox,pcqpcq/sox,Distrotech/sox,cbagwell/sox,pcqpcq/sox,uklauer/sox,CaptainHayashi/sox,cbagwell/sox,pcqpcq/sox,cbagwell/sox,uklauer/sox,davel/sox,uklauer/sox,davel/sox,CaptainHayashi/sox,Distrotech/sox